Glass-ceramic articles have been marketed commercially for over three decades, the first of these being Corning's Code 9606 glass-ceramic radomes which were and still today are supplied by Corning Incorporated, Corning, N.Y. under the trademark PYROCERAM to United States missile manufacturers. Those radomes, containing cordierite as the predominant crystal phase, have a composition coming within the disclosure of U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics.
As is well-recognized in the art, glass-ceramics have customarily been prepared through the controlled crystallization of a precursor glass. Thus, glass-ceramic articles have been conventionally produced following the three general steps below: (a) a glass-forming batch containing a nucleation agent is melted; (b) that melt is cooled to a temperature at least below the transformation range and simultaneously a glass article of a desired configuration shaped therefrom; and (c) that glass article is heat treated in accordance with a predetermined schedule in order to controllably crystallize it in situ. Frequently, the heat treatment is carried out in two stages: (1) the glass is initially heated to a temperature somewhat above its transformation range to develop nuclei therein; and then (2) the nucleated glass is heated to a temperature approaching or even exceeding the softening point thereof to grow crystals on the nuclei. (As employed herein, the expression transformation range is defined as that temperature at which a melt is transformed into an amorphous solid, that temperature typically being deemed to reside in the vicinity of the annealing point of a glass.)
Because a glass-ceramic is derived from the in situ crystallization of a glass article, it is void-free and non-porous. Furthermore, because the customary glass-ceramic is more crystalline than glass, the chemical and physical properties exhibited thereby will be more reflective of the crystal phase(s) present therein than of the parent glass, and the residual glass remaining therein will have a different composition from the original glass, inasmuch as components constituting the crystal phase(s) will have been removed therefrom. To illustrate, the coefficient of thermal expansion and the refractoriness exhibited by a glass-ceramic are typically quite different from those demonstrated by the precursor glass.
As can be appreciated, the presence of a nucleating agent alters the properties which would be displayed where a desired crystal phase is present alone. For example, in some instances the nucleating agent is less refractory than the crystal phase generated, which circumstance diminishes the refractoriness of the glass-ceramic. In other instances, the nucleating agent may become part of the residual glass, thereby resulting in a higher level of glassy phase in the final product. In still other instances, the presence of a nucleating agent can lead to the generation of several crystal phases, thereby diluting the effect of a single desired crystal phase.
Therefore, in a rather recent modification of the basic process for manufacturing glass-ceramic bodies, the following procedure has been devised: First, a glass-forming batch is melted; Second, that melt is cooled to a temperature below the transformation range thereof and simultaneously a glass body of a desired configuration is shaped therefrom; Third, that glass shape is comminuted to a very fine powder (commonly called frit); Fourth, that frit is formed into a desired shape; and Fifth, that shape is heated to a temperature and for a time sufficient to sinter the particles into an integral, non-porous article which, utilizing surface nucleation of the very fine particles, will thereafter be crystallized in situ into a glass-ceramic.
As can be appreciated, articles of complex shapes can be prepared through a variety of conventional ceramic powder processing techniques such as dry pressing, drain casting, rolling, hot pressing, isostatic pressing, jiggering, slip casting, etc. The use of frits has also been found to be especially convenient in providing a glass-ceramic coating or film on a substrate.